MoreLikeThis keeps the lowest-scoring selected terms when the query reaches the boolean clause limit, so the truncated query silently drops the most interesting terms (regression since 4.10.0)
Finding LFV-571Area: queries / MoreLikeThis query construction (MoreLikeThis.createQuery truncation at IndexSearcher.getMaxClauseCount)Priority: lowImpact: silent wrong resultsNovelty: new
AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.
What it shows
You let MoreLikeThis select more terms than the global boolean clause limit, for example setMaxQueryTerms(2000) on a long text under the default limit of 1024, and call like(...). You expect the query, which createQuery deliberately truncates to the clause limit instead of failing, to keep the 1024 most interesting (highest tf*idf) of the selected terms, as it did up to 4.9.1. Instead createQuery pops its queue lowest score first and stops at the first TooManyClauses, so the query holds the 1024 lowest-scoring selected terms and none of the best ones (in the repro, 0 of the 76 top words survive).
The test shows that when MoreLikeThis selects more terms than the boolean clause limit allows, the truncated query keeps the lowest-scoring terms instead of the best ones.
Expected vs actual
Expected [alpha, beta] (tf 4 and 3) under a clause limit of 2 with the default maxQueryTerms; actual [delta, gamma] (tf 1 and 2), with -ea and -da.
Files
Run it
- Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
- Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
- Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
- How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
- Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.
# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar # JUnit Platform console launcher (Maven Central)
javac -d classes -cp "$CP" $(find src -name '*.java')
# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
-cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
execute --select-class org.apache.lucene.queries.mlt.TestMoreLikeThisClauseLimitGradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).
Formal models and scripts this finding's part of the hunt used: queries-mlt-commonterms-reference
Feedback
Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.
Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.